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<div class="titlepage"><div><div><h4 class="title">
<a name="asio.reference.async_read_at.overload3"></a><a class="link" href="overload3.html" title="async_read_at (3 of 4 overloads)">async_read_at
        (3 of 4 overloads)</a>
</h4></div></div></div>
<p>
          Start an asynchronous operation to read a certain amount of data at the
          specified offset.
        </p>
<pre class="programlisting">template&lt;
    typename <a class="link" href="../AsyncRandomAccessReadDevice.html" title="Buffer-oriented asynchronous random-access read device requirements">AsyncRandomAccessReadDevice</a>,
    typename Allocator,
    typename <a class="link" href="../ReadHandler.html" title="Read handler requirements">ReadHandler</a>&gt;
<a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> async_read_at(
    AsyncRandomAccessReadDevice &amp; d,
    uint64_t offset,
    basic_streambuf&lt; Allocator &gt; &amp; b,
    ReadHandler &amp;&amp; handler);
</pre>
<p>
          This function is used to asynchronously read a certain number of bytes
          of data from a random access device at the specified offset. The function
          call always returns immediately. The asynchronous operation will continue
          until one of the following conditions is true:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
              An error occurred.
            </li></ul></div>
<p>
          This operation is implemented in terms of zero or more calls to the device's
          async_read_some_at function.
        </p>
<h6>
<a name="asio.reference.async_read_at.overload3.h0"></a>
          <span><a name="asio.reference.async_read_at.overload3.parameters"></a></span><a class="link" href="overload3.html#asio.reference.async_read_at.overload3.parameters">Parameters</a>
        </h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl>
<dt><span class="term">d</span></dt>
<dd><p>
                The device from which the data is to be read. The type must support
                the AsyncRandomAccessReadDevice concept.
              </p></dd>
<dt><span class="term">offset</span></dt>
<dd><p>
                The offset at which the data will be read.
              </p></dd>
<dt><span class="term">b</span></dt>
<dd><p>
                A <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a>
                object into which the data will be read. Ownership of the streambuf
                is retained by the caller, which must guarantee that it remains valid
                until the handler is called.
              </p></dd>
<dt><span class="term">handler</span></dt>
<dd>
<p>
                The handler to be called when the read operation completes. Copies
                will be made of the handler as required. The function signature of
                the handler must be:
</p>
<pre class="programlisting">void handler(
  // Result of operation.
  const asio::error_code&amp; error,

  // Number of bytes copied into the buffers. If an error
  // occurred, this will be the number of bytes successfully
  // transferred prior to the error.
  std::size_t bytes_transferred
);
</pre>
<p>
                Regardless of whether the asynchronous operation completes immediately
                or not, the handler will not be invoked from within this function.
                Invocation of the handler will be performed in a manner equivalent
                to using <code class="computeroutput">asio::io_context::post()</code>.
              </p>
</dd>
</dl>
</div>
<h6>
<a name="asio.reference.async_read_at.overload3.h1"></a>
          <span><a name="asio.reference.async_read_at.overload3.remarks"></a></span><a class="link" href="overload3.html#asio.reference.async_read_at.overload3.remarks">Remarks</a>
        </h6>
<p>
          This overload is equivalent to calling:
        </p>
<pre class="programlisting">asio::async_read_at(
    d, 42, b,
    asio::transfer_all(),
    handler);
</pre>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2018 Christopher M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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